In many vehicle body applications it is necessary to provide parts having a high endurance, i.e. resistance to fatigue, as measured by the direct flexure or repeated bending tests.
Good results under these tests are indicative of the ability of the part to withstand variations in loading, frequency and amplitude of stresses, and, more generally, a resistance to fluctuations in the location, magnitude and timing of stress applications. Direct flexure or repeated bending tests are generally tests in which the article is subjected to bending from one side to another repeatedly. Such tests thus measure a certain kind of fatigue resistance which is particularly desirable in vehicle body applications and, especially, in applications for vehicular suspensions, e.g. road vehicle suspensions and aircraft landing gear, where the parts are subjected to vibration and load-directional change stresses to an inordinate degree.
It has been known to fabricate parts which may be subjected to such loading from steels having a carbon content of 0.2 to 0.45% by weight, and containing small quantities of alloying components, such as chromium and manganese individually or in combination or in binary combination with other alloying ingredients.
Typical steels, which have been used for this purpose, are CK 45, CK 35, 16 MnCr 5, and 41 Cr 4. Larger parts can be fabricated, for example, from 42 Cr Mo 4.
In general, utilizing the known characteristics of such steel, the design engineer must dimension the part which is to be fabricated to be capable of withstanding the stresses to be taken up by the part. Naturally, where the materials are less than satisfactory, as is the case with the steels mentioned, in resisting fatigue or the type represented by the direct flexure testing, the parts must be made somewhat larger and heavier than might otherwise be desirable.
When the parts are fabricated in a sheet metal construction, i.e. consist of profiled sheet materials, they must be dimensioned to be comparatively large to be able to withstand the applied stresses and take up considerable space. When sheet metal construction is not contemplated and the parts are fabricated by casting, for example, they are comparatively massive.
For optimum vehicle design, the suspended mass must be minimized. Since elements of the type described above are frequently used in the suspension itself, this means that the weight of these elements can be critical.
Thus there is a need to minimize the weight of such elements. However, since the structural integrity of the vehicle depends upon the ability of such elements to withstand said changes in applied stress, overloading and the like, it is imperative that the elements not only be designed to withstand the applied stress or for a considerable margin of safety therewith, but also be subjected to destructive testing of production samples as well as to nondestructive testing of the parts used, to ensure the ability of the parts to withstand overloads. These testing techniques may contribute up to 5% of the cost of these parts and may thus make a significant contribution to the overall cost of the vehicle. In certain cases, these costs can be prohibitive.
Indeed, certainty as to performance of the parts hitherto fabricated from the aforementioned steels can only be obtained by testing each individual part at considerable cost and even then total reliability could not be assured under all circumstances. Since complete testing was not always practical from an economical point of view, the use of such parts was always a compromise between the various conditions set forth above.
The problem was complicated even further by the fact that parts made by conventional techniques from the aforementioned steels, when subjected to surface treatments, e.g. hardening, hard facing and the like, or when subjected to machining, were found to be subject to additional stresses whose effects on the fatigue resistance of the product was not always predictable or satisfactory.